Day_Lecture_VI

Day_Lecture_VI - LECTURE VI I. Haloalkanes Let's begin our...

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1 LECTURE VI I. Haloalkanes Let’s begin our survey of functional groups with a class of compounds that we have already considered from the point of view of nomenclature. A. Nomenclature 1. Recall that the halogen functional group is treated by the IUPAC system as if it were an alkyl group substituted on the main chain. The halogens are alphabetized along with the alkyls. 2. Simpler haloalkanes can be named as derivatives of the corresponding hydrogen halides replacing hydrogen with the name of the simple alkyl radical. 3. Other common names for haloalkanes are encountered for compounds that have more than one halogen such as chloroform, iodoform CHI 3 (a common antiseptic with a characteristic odor), carbon tetrachloride etc. Haloalkanes in which all the hydrogens of the alkane have been replaced by halogens are called perhaloalkanes. Example, perchloroethane. Some of these are used as dry cleaning solvents. Freons are chlorofluorocarbons that have been used as refrigerants, but are implicated in destruction of the ozone in the stratosphere.
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2 B. Remember the trends in the halogens (Column VIIA) of the periodic table. Electronegativity and thus bond polarity decreases from top to bottom. Size and thus bond length increases from top to bottom and fluorine is anomalously small. [See Table 6.1} The boiling points of the haloalkanes reflect two properties, the polarity of the C-X bond and the polarizability of the larger halogens. As a result of these factors, especially the latter, the boiling points of haloalkanes are higher than the corresponding alkanes. The densities of the haloalkanes especially, bromo and iodo, are relatively high compared to the corresponding alkane because the volume of even the larger halogens is much smaller than that of a methyl group, but their mass is much greater. C. With the exception of the C-F bond all the C-X bonds are weaker than C-H bonds and therefore haloalkanes are much more reactive than alkanes. [Table 6.1] D. We begin our study of the reactions of haloalkanes with one particular reaction, nucleophilic substitution and one particular kind of nucleophilic substitution that takes place with primary haloalkanes and to a limited extent with secondary haloalkanes, but not with tertiary haloalkanes. Let’s approach these reactions from a very fundamental point of view. 1. Virtually all the reactions of haloalkanes that we will study are heterolytic reactions and reflect the polarity of the C-X bond. Whether the C-X bond is broken to start a reaction off or whether
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3 at some later stage of a reaction the C-X bond is transformed into another kind of bond, the polarity of the bond controls the course of the reaction. 2.
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This note was uploaded on 04/04/2008 for the course CHEM 307 taught by Professor Several during the Spring '08 term at Rutgers.

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Day_Lecture_VI - LECTURE VI I. Haloalkanes Let's begin our...

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